Visual Inspection

Visual inspection (VI) involves inspecting a structure visually and looking for signs of corrosion or other damage [10]. Visual inspection can be used virtually through all stages of product development and construction, starting from raw material and ending with the finished product. The VI can be used for inspecting gas pipes, tanks, building, bridges,

power generating turbines, and power plants. There are several factors that can affect the outcome of VI such as condition of the object under test, environmental conditions and the skill set for the inspector [8-10]. The condition of the object under test includes material, shape, size, access, and surface condition. The shape of the object dictates the amount of light, and/or orientation of viewing angle to insure that all surfaces of the object are examined. The size of the object directly influences the pattern, direction and speed of the examination. Surface condition such as cleanness, rust and contamination may prohibit or limit the inspection process and results.

Environmental conditions include lighting, temperature and weather. Sufficient amount of lighting should be available to ensure exposure of the entire surface of the object under test. On the other hand, an inadequate amount of light could inhibit the object's attributes to be revealed; for example because of shadowing. Excessive light (brightness), however, could also inhibit exposing the attributes of the object under test because excessive reflection reduces the ability of the eye to see small details [10]. If the object under test is inspected under high environmental temperature, the temperature may cause a distortion to the field of view because of heat waves. The inspector’s experience and health condition (stress, tension and fatigue) may adversely affect the perception of the eye and, consequently, affect the inspection results.

Although there are many tools, such as cameras, magnifiers, rules, micrometers, monitors and scopes, that can be used to aid during VI, the inspector's eyes are the primary inspection tool. Since VI depends primarily on the eye for inspection, the eye is required to be in continuous movement (back and forth) during the inspection [10]. This rapid movement of the eye, after a long time of inspection, could cause muscle fatigue in the eye and affect the inspection results. Another variable that could limit the VI is

Although it is useful, VI is limited to visible signs of deterioration that appears at the outer surface of the material such as corrosion, erosion, cracks, voids, wear and fatigue [8-10]. Also, depending on the structure of the object under test, a deterioration at the outer surface of the object may or may not be a sign of internal gross damage.

Unfortunately, visual inspection cannot detect non-visible or hidden corrosion, especially for cable-stayed bridge, because the steel cable is covered under one or more protective layers. However, even if the VI is used to inspect the cables of bridge structures, all protective layers (polyethylene and grout) will have to be removed which is expensive but not very practical. However, the visual inspection of cables in bridges is sometimes performed in a few local areas to achieve an understanding of the condition of the cables.

In such cases, the wires or strands that are bundled together to form a bridge cable are pried open with a tool (i.e., wooden-wedges) so they can be inspected. This can only be done to a limited portion of the cable (a few feet), making VI impractical, inefficient and very costly.

2.3 Radiography

Radiographic methods use radiation beams to detect defects in the materials under inspection [8-12]. Typically, the source of the beam is either Beta or Gamma particles. X-rays are produced by accelerating electrons at high speed under high voltage to strike a filament material to produce photons [8-10, 12]. The photons can then be directed at the

right angle towards the target under test using a special apparatus. The energy of the produced photons is given by [12]:

(2.1)

Figure 2.1 shows a general setup for an X-ray inspection method with a material under inspection. X-rays or Gamma-rays enter the material under test from one side and some or all exit from the other side. The beam usually travels in a straight line. However, it goes under scattering, and absorption process; the amount of scattering and absorption depends on the material's molecular structure. The beams that penetrate through the material are detected and recorded either on a real-time digital monitoring receiver or on a special radiographic detector to produce 2D images [13-19].

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Figure 2.1 X-ray Method: X-ray diagram showing radiation beams penetrating an object and being collected at the receiver end to generate an image.

Radiation Source (High voltage)

Object

Detector

Images produced from the X-ray exposure are usually gray-scale with varying intensity based on the amount of radiation that is collected at the receiver side. These images are usually poor quality and are not sufficient to clearly show the presence of flaws. Also, the orientation of scanning has to be taken into consideration when imaging steel with with a small loss of section or defect since the defect may be masked by the total mass of the steel. This will make it difficult to detect small defects in 2-D X-ray images. X-ray systems could be portable scanning devices, but special protocol has to be followed to ensure the safety of the operators and other personnel during the radiation exposure process [13, 14, 16, 18]. Although there are advantages for using X-ray systems, including detecting internal defects [9], they require a great amount of radiation to penetrate thick and dense materials such as steel cables in cable supported bridges. Such applications of X-ray technique will require a large source of high voltage power to meet the demand for higher radiation and operation of the system. Providing such high power at the bridge site or in the field and installation of a large testing system on a bridge cable can become difficult. These make X-ray technique a non-field-worthy option for inspection of bridge cables.